Selective catalytic reduction (“SCR”) systems are used to reduce emissions in the exhaust of diesel engines. One method applied by SCR systems involves the addition of a gaseous or liquid reductant (also referred to as a reducing agent), such as ammonia or urea, to the exhaust gas stream from an engine. The reductant is thereby absorbed onto a catalyst where the reductant reacts with nitrogen oxides (“NOx”) in the exhaust gas to form water vapor and nitrogen.
The amount of reduction added to the NOx conversion system depends on the amount of NOx exhaust that is produced by the engine. NOx conversion systems therefore measure the generated NOx and then determine a corresponding amount of reductant added to the exhaust stream based on the pressure differential between the exhaust stream and the reductant supply system. A physical or virtual sensor may detect the NOx concentration of the exhaust flow, allowing the SCR to determine the appropriate dosage of ammonia to be injected into the exhaust stream. A reductant delivery system can then deliver the appropriate dosage of ammonia by adjusting certain parameters in the reduction delivery system by way of a pump and a control valve. The flow rate for the dosage of reductant can be calculated by a one-dimensional fluid equation, or a one-dimensional gas equation using signals from various sensors in the system. For example, the system may use information obtained from pressure sensors, temperature sensors, and the flow path cross sectional area of the reductant delivery system.
For a variety of reasons, however, the actual amount of the reductant supplied into the exhaust stream may differ from the calculated or intended amount. That is, the actual flow of reductant into the exhaust stream may not be accurately determined by the equipment. For example, one of more of the pressure sensors in the exhaust gas purification system may degrade, malfunction, or drift causing faulty measurements. Also, clogs and corrosions may occur in the flow path of the reductant delivery or exhaust systems. These issues may result in a faulty measurement thereby causing the system to improperly calculate in the amount of reductant dosage that is actually supplied to the NOx exhaust. As a result of such a faulty dosage, the NOx conversion efficiency will decrease and the NOx emissions will increase.
Employing a diagnostics system can ensure that the actual reductant dosage matches the desired dosage. A diagnostics system can monitor the actual flow of the reductant to ensure that the actual flow matches the desired flow necessary for proper NOx conversion. Performing diagnostics can detect when the calculated flow rate of the reductant differs from the actual flow rate of the reductant and help determine the cause for the discrepancy.